<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="vue.js"></script>
</head>
<body>
   <div id="app">
    <button @click="jia(num)">加</button>
    <button @click="jian(num)">减</button>
    <p>{{num}}</p>
   </div>
    <script>
let vm=new Vue({
    el:"#app",
    data(){
        return{
            num:3
        }
    },
    methods:{
        jia(val){
            this.num++
        },
        jian(val){
            this.num--
        }
    }
})
    </script>
</body>
</html>